1. Field of the Invention
The present invention relates to an ink composition, and more particularly to an ink composition that can be preferably used for ink jet recording.
2. Background Art
Ink jet recording is a printing method wherein droplets of an ink are ejected and deposited onto a recording medium, such as paper, to conduct printing. This method has a feature that an image having high resolution and quality can be printed at a high speed by means of a relatively inexpensive apparatus.
In general, the ink composition used in the ink Jet recording comprises a water-soluble dye or a pigment, a water-soluble organic solvent, and water. One of properties required of the ink is to realize an image suffering from no significant feathering or bleeding. In particular, in multi-color printing, one color ink frequently comes into contact with another ink. Mixing of two inks of different colors on a recording medium, or the creation of bleeding on the recording medium yields an unfavorable color, resulting in a deterioration in resolution of the image, color separation, sharpness of edge, and color purity. Therefore, realizing an image having no significant feathering or bleeding is an important property required of the ink composition.
Further, in recent years, utilization of pigments as a colorant has been attempted. Pigments, as compared with dyes, possess superior waterfastness and lightfastness. Since, however, the pigment is left on a recording medium to form a pixel thereon, it has been often pointed out that the formed image has inferior rubbing/scratch resistance. Further, the pigment is essentially nonaqueous and hence should be stably dispersed in the ink composition. In order to stably disperse the pigment, several methods have been proposed wherein a resin is added to the ink composition. For example, Japanese Patent Publication No. 1426/1987 discloses an ink comprising a pigment and a resin emulsion dispersed in water, Japanese Patent Laid-Open No. 157668/1980 discloses a dispersion of a pigment in a water-insoluble, resin emulsion dispersion, Japanese Patent Laid-Open No. 217088/1989 discloses an ink using an emulsion having a specific film-forming temperature, Japanese Patent Laid-Open Nos. 60068/1991 and 18462/1992 likewise disclose an ink using a resin emulsion. Further, Japanese Patent Laid-Open Nos. 147859/1981 and 147860/1981 and Japanese Patent Publication No. 5703/1992 propose an aqueous dispersion type pigment ink using a polymeric dispersant and a water-soluble organic solvent. Unlike the dye which is penetrated into and fixed on a recording medium, the pigment is basically anchored and fixed onto the surface of the recording medium, often resulting in the formation of an image having poor rubbing/scratch resistance. In order to improve the fixability of the pigment onto the recording medium, addition of a resin component to the ink composition to strongly anchor the pigment onto the surface of the recording medium has been proposed in the art.
The addition of the resin to the pigment-based ink composition often leads to increased viscosity of the ink composition and, in some cases, renders the ink composition unsuitable for the ink jet recording. Further, the addition of the resin often develops such an unfavorable phenomenon that the properties of the ink composition are varied, causing the ink composition to wet a nozzle plate of an ink jet recording head. This has led to problems associated with ejection stability, such as inability of the ink composition to be ejected and ink droplet trajectory directionality. Further, the addition of the water-soluble resin to the ink often lowers the waterfastness of the yielded image or nozzle clogging of the recording head due to an increase in viscosity of the ink composition as a result of vaporization of water.
On the other hand, a new ink jet recording method has been recently proposed which comprises applying a polyvalent metal salt solution onto a recording medium and then applying an ink composition containing a dye having at least one carboxyl group (for example, Japanese Patent Laid-Open No. 202328/1993). According to this method, polyvalent metal ions combine with the dye to form an insoluble composite which can provide a high-quality image having waterfastness and free from color bleeding (for example, Japanese Patent Laid-Open No. 106735/1994).
Further, an ink jet recording method has been proposed wherein a color ink containing a surfactant or a penetrable solvent and a salt for imparting a penetrating property is used in combination with a black ink which cooperates with the salt to cause thickening or agglomeration, thereby providing a high-quality color image having high image density and free from color bleeding (Japanese Patent Laid-Open No. 106735/1994). More specifically, in this method, two liquids, i.e., a first liquid containing a salt and a second liquid of an ink composition, are printed to provide a good image.
Furthermore, other ink jet recording methods wherein two liquids are printed have been proposed, for example, in Japanese Patent Laid-Open No. 240557/1991 and No. 240558/1991.
In the above ink jet recording methods wherein two liquids are printed, a further improvement in properties has been desired in the art. One of the properties to be further improved is fixability of the colorant. Ink jet recording, plain papers and recycled papers are in many cases used in addition to specialty recording media. For many of these recording media, ink is likely to penetrate in the recording medium. For this reason, even though a high-quality image is formed on a certain medium, feathering or color bleeding is often created on other media. A reduction in this problem has been desired in the art. Secondly, a reduction in uneven printing is required. The uneven printing refers to a variation in color density in the print derived from localization of a colorant on the paper. Although the uneven printing poses no significant problem in printing of letters of regular size, it is a serious problem in applications where figures, graphs and the like should be printed.